SELECTION PROCESS FOR HAZARDOUS AREA CABLE GLANDS ACCORDING TO IEC
Concerning the subject of cable glands to maintain integrity of type of protection Flameproof Enclosures "d" using direct cable entry into the flameproof enclosures, special selection criterions have to be considered as defined in Section 10 of IEC Standard IEC 60079-14 "Electrical apparatus for explosive gas atmospheres Part 14: Electrical installations in hazardous areas (Other than mines)"
In order to achieve compliance with the prevailing Installation Code of Practice and in particular IEC 60079-14, it is necessary to evaluate the function of the equipment, the cable gland, and the cable, and in order to satisfy the conditions of the applicable standards compatibility of all three with each other must be verified. Section 10 of IEC 60079-14 "Additional requirements for type of protection "d"- Flameproof enclosures" for the selection of cable glands is required to be followed, and this sets out some specific rules to ensure integrity and safe operation of the installed equipment.
Cable Entries into Ex d Enclosures
Two situations can be considered, direct cable entry and indirect cable entry
Indirect Cable Entries into Ex d Enclosures
In the case of indirect cable entry, this may be achieved by a separate terminal chamber, where the cable entries can be found and only the (looping) cable conductors are terminated in the terminal block, where no source of ignition exists in this terminal chamber. The terminal chamber may offer Ex d or Ex e form of protection, and is separated from the main enclosure, with the internal wiring passing through line barriers or bushings before connecting to the equipment side of the terminal block. The termination and wiring of the incoming field cables would thereby not normally require the adoption of compound sealing cable glands in this case, unless there is a risk of gas migration through the interstices of the cable and the transmission of gas to the opposite end of the cable needs to be prevented. An example of this type of equipment configuration is shown in the photograph to the right.
Selection Process - According to IEC 60079 : 14
The following flow chart can only be followed as part of the IEC selection process after physical evaluation of the cable
The cable entry system shall comply with the following:
a) Cable entry device in compliance with IEC 79-1 "Construction and verification test of flameproof enclosures of electrical apparatus" and particular type of cable intended for use with that device,
b) thermoplastic, thermosetting or elastomeric cable which is substantially compact and circular, has extruded bedding and fillers, if any, are non-hygroscopic, may utilize flameproof cable entry devices, incorporating a sealing ring selected in accordance with figure 1,
Figure 1: Selection chart for cable entry devices into flameproof enclosures for cables complying with item b) of Section 10.4.2.b.
On condition the cable gland is not certified as part of the equipment but tested and certified as a separate component and the used cable is substantially compact and circular the selection chart above taken from section 10 of EN/ IEC60079-1 can be used.
Cable Construction - Should be Round
In order to comply with IEC installation standards, cable glands using elastomeric sealing rings as a means of maintaining the Flameproof protection method can only be used if the cable selected is :-
"Substantially compact and circular with an extruded bedding, and if any fillers are used they are Non-Hygroscopic"
This is clearly not always the case with cables used in hazardous areas.
But the cable must play a part in the safety of the installation, even in the case of indirect cable entry, when gas migration must be avoided.
e.g., where cables run across two zones, or indeed from a hazardous area into a safe area.
Sample IEC Cable Configurations
Which type is suitable for use with Flameproof Ex d equipment when a cable gland with an elastomeric sealing ring would be considered?
Cable A is not suitable to apply a Flameproof sealing ring as this cable is the incorrect shape, and unless the cable is round the sealing ring will not be able to make an effective seal on the cable.
Cables B, D & E are not suitable to apply a Flameproof sealing ring, as the white areas represent a gap or void in the cable whereby there is either no inner cable sheath, or extruded bedding, or suitable fillers are absent. In this case no protection to the interstices of the cable can be offered by a sealing ring.
Cable C is the only one of the five sample cables illustrated which could be selected as correctly meeting the IEC 60079-14 criteria, as it has an extruded inner cable bedding and there is no gas migration path between the conductors.
Equally, if the cable is not adequately filled, and allows the passage of air or gas to flow along the cable length then there would be no protection to the inner part of the cable when an elastomeric sealing ring is used. In this case a compound barrier type cable gland is the only safe solution and this is needed to maintain the integrity of the equipment as explained above, and prevent gas migration from equipment to equipment, or hazardous areas to safe areas.
Ex d / Ex e IIC & Class I Division 1 - Compound Barrier Gland for Armoured Cables
In conclusion it should be acknowledged that when it comes to the correct selection, deployment or installation and maintenance of cable glands in hazardous areas, referring merely to the product certification is not enough, and the guidelines laid down in the available installation code(s) of practice should be strictly observed.
The code of practice for any installation will play a vital part in the overall safety of the plant and failure to comply with it will usually lead to non-conformities and incompatibilities taking place that will inevitably compromise some aspect of safety in the ongoing operations.